1. Field of the Invention
The present invention relates to a dental shade system for measuring natural tooth color, by classifying natural tooth colors in terms of three dimensional coordinates of Value, Chroma, and Hue and by arranging dental color samples (shades) equidistantly in vision.
2. Description of the Conventional Art
Dental shade systems (in other words, dental shade guides) are dental systems composed of color samples necessary to meet a patient's satisfaction by matching the color of dental prostheses including artificial teeth to the patient's natural tooth color, improving prostheses esthetically. In the process of fabricating dental prostheses, information of the patient's tooth color is required to be included in the instructions written in a prescription card sent from the dentist to the laboratory technician. Therefore, in a dental clinic, it is indispensable to be able to select the color which matches the patient's natural tooth color and send corresponding information to the laboratory. As patients' interest in dental esthetics has increased, the importance of precise information has rapidly increased.
As is generally known, the colors that human eyes sense are described by three dimensional coordinates; Value (lightness level), Chroma (color saturation), and Hue (color). These coordinates have been well known as the Munsell system (Munsell, 1905), by reason of its color samples being arranged with perceptually even spacing. However, with the development of color science, the existence of the inherent chromatic distortions in the color space of the Munsell system became clarified. Spacing the color samples requires a non distorting color space such as the more recently developed and internationally accepted colorimetric for reflective materials, CIELAB (CIE 1978).
However, conventional dental shade guides have not been arranged logically or scientifically and do not even correspond to measured tooth color. Shade guides of all dental restorative materials are based on the long established porcelain shade guides which evolved to represent the available shades of porcelain teeth. The shades developed by a process of popular selection by which shades perceived to be nearer tooth color were added and the least popular eliminated. This concept has not changed since the introduction of porcelain over two hundred years ago.
The color of teeth is limited from yellowish to reddish and occupies about a 1% region of a total color space. Moreover, existing dental shade guides have contained only 6.about.12% of the 1% region of the total color space mentioned above. In addition, since these dental shade guides have been selected by experience, the distribution of their colors in color space has not been uniform. Accordingly, the result of their use has been incorrect and not effective, so it has been difficult to visually match the color of prostheses to a patient's natural tooth color.
To resolve this problem, in recent years, a dental shade system applying color science was developed by Hall, evaluating existing methods and problems and quantifying the difficulties of color matching. A tooth color order system was developed resulting in the construction of a tooth color atlas assembled for easy use offering the accurate measurement of tooth color and the potential of perfect color matching (Hall 1991 and U.S. Pat. No. 5,498,157).
In the dental shade system developed by Hall, the color samples are arranged like a lattice in color space at equal intervals which are near the threshold value of the ability of human sight to distinguish colors. They consist of Values having 5 steps (marked as 1, 2, 3, 4, and 5 in order of lightness), Chromas having 3 steps (marked as 1, 2, and 3, in order of weak, medium, and strong clearness), and Hues having 3 steps (marked as L, M, and R in order of yellowish, medium, and reddish), totaling 5.times.3.times.3=45 color samples as the basic composition of the system. By this composition, a first scientifically reasonable dental shade system which covers the total region of the color distribution of human natural tooth in color space has been actualized. In Table 1, the basic composition of the color sample arrangement of the Hall dental shade system 111 is shown. In the basic composition of the color sample arrangement of the Hall dental shade system shown in Table 1, the colors of each color sample are marked according to Hall's notation. For example, 3M/2 means that the Value of the color sample is 3, the Chroma is 2, and the Hue is M.
The occurrence rate of tooth color in color space is not uniform. When we mark Values having 5 steps as V1, V2, V3, V4, and V5 in order of lightness (corresponding to 1, 2, 3, 4, and 5 mentioned above), the occurrence rate of V1 is 2%, V2's is 26%, V3's is 50%, V4's is 20%, and V5's is 2%, differing largely with each other. Since a similar situation exists for the case of combining Chroma and Hue, it can not be said to be best, in view of a cost versus effect ratio, to compose a dental shade system with many color samples paying attention only to the region of color space that tooth color occupies.
For such a practical reason, Hall reduced the number of color samples from the 45 mentioned above to 26, by excluding those color samples the occurrence rates of which are low. As the result, as shown in FIG. 5, the Hall dental shade system 111 is constituted of 11 bundles which are inserted into the color sample rack 114 arranged horizontally and on each bundle two or three color samples 113 are attached in order and a handle 112 is attached at the bottom.
In Table 2, the practical composition of the color sample arrangement of the Hall dental shade system 111 is shown. In Table 2, x marks show 7 among 19 color samples which are excluded due to their low occurrence rates. (The other 12 color samples in rows 1L, 1R, 5L, 5R which are excluded due to their low occurrence rates, are omitted from Table 2).
In relation to the order for visually selecting the color sample the color of which matches the natural tooth color from among the color samples of the dental shade system 111, Hall recommends selecting it in the order of Value, Chroma, and Hue. In the practical composition of Hall dental shade system 111 shown in Table 2, the reason for the color samples 113 having weak Chroma (1 or 1.5) being arranged in the first line 115 (front line 115 in FIG. 5: upper side in the figure) is, that if the color samples had strong Chroma, it would be difficult to decide the Value for the natural tooth by bringing the color sample rack 114 to the patient's natural tooth. In addition, as shown in FIG. 5, the tops of the color samples having medium Hue (M) among the color samples arranged in the first line 115 are projected slightly higher (in FIG. 5) than the tops of the neighboring color samples in the first line 115 so that the visual appearance is not disturbed by the neighboring color samples having yellowish Hue (L) or reddish Hue (R). Moreover, in the practical composition of the Hall dental shade system 111 shown in Table 2, some of Chromas 1 or 3 are replaced by 1.5 or 2.5 where latter marks (1.5 and 2.5) indicate intermediate Chromnas, respectively. The reason for the above replacements is to revise non-uniformity of the distance in color space between residual color samples which has resulted from having color samples with low occurrence rates excluded, as mentioned above.
As described heretofore, the method of deciding the color in terms of Value, Chroma, and Hue by means of comparing the color samples visually with the object, is a classical one known together with the name of the Munsell system. However, the method of this type is, even in the present day, said to be the most superior dental clinical method when compared to modem colorimetrics utilizing electronics, for the reason that the method is extremely low cost, does not need a standard light source or a dark room, and can perform colorimetry visually with high sensitivity.